Apparatus and method for decoding in a hierarchical, modulation system

ABSTRACT

A satellite receiver receives a hierarchical modulation based received signal, which has at least an upper layer (UL) and a lower layer (LL), and simultaneously recovers therefrom data conveyed in the UL signal and data conveyed in the LL signal.

BACKGROUND OF THE INVENTION

The present invention generally relates to communications systems and,more particularly, to satellite-based communications systems.

As described in U.S. Pat. No. 5,966,412 issued Oct. 12, 1999 toRamaswamy, hierarchical modulation can be used in a satellite system asa way to continue to support existing legacy receivers yet also providea growth path for offering new services. In other words, abackward-compatible hierarchical modulation based satellite systempermits additional features, or services, to be added to the systemwithout requiring existing users to buy new satellite receivers. In ahierarchical modulation based communications system, at least twosignals, e.g., an upper layer (UL) signal and a lower layer (LL) signal,are added together to generate a synchronously modulated satellitesignal for transmission. In the context of a satellite-basedcommunications system that provides backward compatibility, the LLsignal provides additional services, while the UL signal provides thelegacy services, i.e., the UL signal is, in effect, the same signal thatwas transmitted before—thus, the satellite transmission signal cancontinue to evolve with no impact to users with legacy receivers. Assuch, a user who already has a legacy receiver can continue to use thelegacy receiver until such time that the user decides to upgrade to areceiver, or box, that can recover the LL signal to provide theadditional services.

In a hierarchical modulation receiver, the received signal issequentially decoded, i.e., the received signal is first processed torecover data conveyed in the UL signal, which is then used to recoverdata conveyed in the LL signal. In particular, the received signal isfirst demodulated and the upper layer (UL) is decoded therefrom—thisprovides the data conveyed in the UL. This data, i.e., the decoded ULsignal, is then re-encoded to provide a re-encoded UL signal. There-encoded UL signal is then subtracted from the demodulated receivedsignal to uncover the LL signal, which is then decoded to recover thedata conveyed therein. Hence, the demodulation and decoding of the LLsignal depends on the UL signal.

SUMMARY OF THE INVENTION

We have observed that sequential decoding in a hierarchical modulationreceiver not only adds complexity to the receiver, but also may degradereceiver performance if the recovered upper layer (UL) signal doesn'tmatch the originally transmitted UL signal due to errors in the ULdecoding process—thus, introducing errors into the recovered LL signal.Therefore, and in accordance with the principles of the invention, areceiver receives a hierarchical modulation based received signal, whichcomprises at least a first signal layer and a second signal layer, andsimultaneously recovers therefrom data conveyed in the first signallayer and data conveyed in the second signal layer.

In an embodiment of the invention, a satellite communications systemcomprises a transmitter, a satellite transponder and a receiver. Thetransmitter transmits an uplink hierarchical modulation based signal tothe satellite transponder, which broadcasts the hierarchical modulationbased signal downlink to a receiver. The receiver processes -thereceived hierarchical modulation based signal such that data conveyed inthe UL and data conveyed in the LL are simultaneously recoveredtherefrom.

In another embodiment of the invention, a satellite communicationssystem comprises a transmitter, a satellite transponder and a receiver.The transmitter transmits an uplink hierarchical modulation based signalto the satellite transponder, which broadcasts the hierarchicalmodulation based signal downlink to a receiver. The receiver processesthe received hierarchical modulation based signal such that the UL andthe LL are processed independently of each other.

In another embodiment of the invention, a receiver for receiving ahierarchical modulation based signal comprising at least a first signallayer and a second signal layer constructs a look-up table of softmetric values. In particular, the receiver receives a training signalfrom an endpoint and calculates soft metric values as a function of acombined signal space and the received training signal, wherein thecombined signal space is a combination of a signal space of the firstsignal layer and a signal space of the second signal layer. The receiverthen stores the calculated soft metric values in the look-up table.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an illustrative satellite communications system embodyingthe principles of the invention;

FIG. 2 shows an illustrative block diagram of a transmission paththrough satellite 15 of FIG. 1;

FIG. 3 shows an illustrative embodiment for implementing hierarchicalmodulation in transmitter 5 of FIG. 1;

FIG. 4 show an illustrative symbol constellation for use in the upperlayer and the lower layer;

FIG. 5 shows an illustrative symbol constellation for a hierarchicalmodulation based signal;

FIG. 6 shows another illustrative embodiment for implementinghierarchical modulation in transmitter 5 of FIG. 1;

FIG. 7 shows an illustrative block diagram of a receiver in accordancewith the principles of the invention;

FIG. 8 shows an illustrative block diagram of simultaneousdemodulator/decoder 320 of FIG. 7 in accordance with the principles ofthe invention;

FIG. 9 shows an illustrative block diagram of demodulator 330 of FIG. 8;

FIG. 10 shows an illustrative signal space;

FIG. 11 shows an illustrative log-likelihood look-up table in accordancewith the principles of the invention;

FIG. 12 shows an illustrative symbol constellation;

FIGS. 13 and 14 illustrate log-likelihood calculations;

FIG. 15 shows an illustrative flow chart for use in receiver 30 of FIG.1; and

FIG. 16 shows another illustrative embodiment in accordance with theprinciples of the invention.

DETAILED DESCRIPTION

Other than the inventive concept, the elements shown in the figures arewell known and will not be described in detail. Also, familiarity withsatellite-based systems is assumed and is not described in detailherein. For example, other than the inventive concept, satellitetransponders, downlink signals, symbol constellations, a radio-frequency(rf) front-end, or receiver section, such as a low noise blockdownconverter, formatting and source encoding methods (such as MovingPicture Expert Group (MPEG)-2 Systems Standard (ISO/IEC 13818-1)) forgenerating transport bit streams and decoding methods such aslog-likelihood ratios, soft-input-soft-output (SISO) decoders, Viterbidecoders are well-known and not described herein. In addition, theinventive concept may be implemented using conventional programmingtechniques, which, as such, will not be described herein. Finally,like-numbers on the figures represent similar elements.

An illustrative communications system 50 in accordance with theprinciples of the invention is shown in FIG. 1. Communications system 50includes transmitter 5, satellite channel 25, receiver 30 and television(TV) 35. Although described in more detail below, the following is abrief overview of communications system 50. Transmitter 5 receives anumber of data streams as represented by signals 4-1 through 4-K andprovides a hierarchical modulation based signal 6 to satellitetransmission channel 25. Illustratively, these data streams representcontrol signaling, content (e.g., video), etc., of a satellite TV systemand may be independent of each other or related to each other, or acombination thereof. The hierarchical modulation based signal 6 has Klayers, where K≧2. It should be noted that the words “layer” and “level”are used interchangeably herein. Satellite channel 25 includes atransmitting antenna 10, a satellite 15 and a receiving antenna 20.Transmitting antenna 10 (representative of a ground transmittingstation) provides hierarchical modulation based signal 6 as uplinksignal 11 to satellite 15. Referring briefly to FIG. 2, an illustrativeblock diagram of the transmission path through satellite 15 for a'signalis shown. Satellite 15 includes an input filter 155, a traveling wavetube amplifier (TWTA) 165 and an output filter 175. The uplink signal 11is first filtered by input filter 155, then amplified for retransmissionby TWTA 165. The output signal from TWTA 165 is then filtered by outputfilter 175 to provide downlink signal 16 (which is typically at adifferent frequency than the uplink signal). As such, satellite 15provides for retransmission of the received uplink signal via downlinksignal 16 to a broadcast area. This broadcast area typically covers apredefined geographical region, e.g., a portion of the continentalUnited States. Turning back to FIG. 1, downlink signal 16 is received byreceiving antenna 20, which provides a received signal 29 to receiver30, which, in accordance with the principles of the invention,demodulates and simultaneously decodes received signal 29 to provide,e.g., content to TV 35 for viewing thereon. It should be noted thatalthough not described herein, transmitter 5 may further predistort thesignal before transmission to compensate for non-linearities in thechannel.

An illustrative block diagram of a hierarchical modulator for use intransmitter 5 is shown in FIG. 3. Hierarchical modulation is simplydescribed as a synchronous modulation system where a lower layer signalis synchronously embedded into an upper layer signal so as to create ahigher -order modulation alphabet. In the remainder of this descriptionit is illustratively assumed that there are two data streams, i.e., K=2.It should be noted that the invention is not limited to K=2 and, infact, a particular data stream such as signal 4-1 may already representan aggregation of other data streams (not shown).

In FIG. 3, the hierarchical modulation transmitter comprises UL encoder105, UL modulator 115, LL encoder 110, LL modulator 120, multipliers (oramplifiers) 125 and 130, combiner (or adder) 135 and up converter 140.The upper layer (UL) path is represented by UL encoder 105, UL modulator115 and amplifier 125; while the lower layer (LL) path is represented byLL encoder 110, LL modulator 120 and amplifier 130. As used herein, theterm “UL signal” refers to any signal in the UL path and will beapparent from the context. For example, in the context of FIG. 3, thisis one or more of the signals 4-1, 106 and 116. Similarly, the term “LLsignal” refers to any signal in the LL path. Again, in the context ofFIG. 3, this is one of more of the signals 4-2, 111 and 121. Further,each of the encoders implement known error detection/correction codes(e.g., convolutional or trellis codes; concatenated forward errorcorrection (FEC) scheme, where a rate ½, ⅔, ⅘ or 6/7 convolutional codeis used as an inner code, and a Reed Solomon code is used as an outercode; LDPC codes (low density parity check codes); etc.). For example,typically UL encoder 105 uses a convolutional code or a short blockcode; while LL encoder 110 uses a turbo code or LDPC code. For thepurposes of this description it is assumed that LL encoder 110 uses anLDPC code. In addition, a convolutional interleaver (not shown) may alsobe used.

As can be observed from FIG. 3, signal 4-2 is applied to LL encoder 110,which provides an encoded signal 111 to LL modulator 120. Likewise,signal 4-1 is applied to UL encoder 105, which provides an encodedsignal 106 to UL modulator 115. Encoded signal 106 represents N bits persymbol interval T; while encoded signal 111 represents M bits per symbolinterval T, where N may, or may not, equal M. Modulators 115 and 120modulate their respective encoded signals to provide modulated signals116 and 121, respectively. It should be noted that since there are twomodulators, 115 and 120, the modulation can be different in the UL pathand the LL path. Again, for the purposes of this description it isassumed that the number of UL encoded data bits is two, i.e., N=2, andthat UL modulator 115 generates a modulated signal 116 that lies in oneof four quadrants of a signal space. That is, UL modulator 115 maps twoencoded data bits to one of four symbols. Similarly, the number of LLencoded data bits is also assumed to be two, i.e., M=2, and LL modulator120 also generates a modulated signal 121 that lies in one of fourquadrants of the signal space. An illustrative symbol constellation 89for use in both the UL and the LL is shown in FIG. 4. It should be notedthat signal space 89 is merely illustrative and that symbolconstellations of other sizes and shapes can be used.

However, the output signals from UL modulator 115 and LL modulator 120are further adjusted in amplitude by a predefined UL gain and apredefined LL gain via amplifiers 125 and 130, respectively. It shouldbe noted that the gains of the lower and upper layer signals determinethe ultimate placement of the points in the signal space. For example,the UL gain may be set to unity, i.e., 1, while the LL gain may be setto 0.5. The UL signal and the LL signal are then combined via combiner,or adder, 135, which provides combined signal 136. Thus, the modulatorof FIG. 3, e.g., the amplifiers 125 and 130, along with combiner 135,effectively further rearranges and partitions the signal space such thatthe UL signal specifies one of the four quadrants of the signal space;while the LL signal specifies one of a number of subquadrants of aparticular quadrant of the signal space as illustrated in FIG. 5 bysignal space 79.

In effect, the resulting signal space 79, also referred to herein as thecombined signal space 79, comprises 16 symbols, each symbol located at aparticular signal point in the signal space and associated with aparticular four bits. For example, symbol 83 is associated with the fourbit sequence “01 01”. The lower two bit portion 81 is associated withthe UL and specifies a quadrant of signal space 79; while the upper twobit portion 82 is associated with the LL and specifies a subquadrant ofthe quadrant specified by two bit portion 81. It should be noted thatsince the UL signal identifies the quadrant, the LL signal effectivelylooks like noise on the UL signal. In this regard, combined signal space79 is representative of the concept and the distances between symbolstherein is not to scale. Returning to FIG. 3, the combined signal 136 isapplied to up converter 140, which provides multi-level modulated signal6 at the appropriate-transmission frequency. Turning briefly to FIG. 6,another illustrative embodiment for implementing hierarchical modulationin transmitter 5 is shown. FIG. 6 is similar to FIG. 3 except thathierarchical modulator 180 performs the mapping of the lower layer andupper layer bits into the combined signal space. For example, the upperlayer may be a QPSK (quadrature phase-shift keying) signal space, whilethe lower layer is a BPSK (binary phase-shift keying) signal space.

As noted above, after reception of the downlink signal 16 by receivingantenna 20, receiver 30 demodulates and decodes received signal 29 toprovide, e.g., content to TV 35 for viewing thereon. An illustrativeportion of receiver 30 in accordance with the principles of theinvention is shown in FIG. 7. Receiver 30 includes front end filter 305,analog-to-digital converter 310 and simultaneous demodulator/decoder320. Front end filter 305 down-converts and filters received signal 29to provide a near base-band signal to A/D 310, which samples the downconverted signal to convert the signal to the digital domain and providea sequence of samples 311 (also referred to as hierarchical signal 311 )to simultaneous demodulator/decoder 320. The latter performsdemodulation of hierarchical signal 311 and, in accordance with theprinciples of the invention, simultaneous, or independent, decoding ofthe resulting demodulated signal to provide a number of output signals,321-1 to 321-K, representative of data conveyed by hierarchical signal311 on the K layers. Data from one or more of these output signals areprovided to TV set 35 via signal 31. (In this regard, receiver 30 mayadditionally process the data before application to TV set 35 and/ordirectly provide the data to TV set 35.) In the following example thenumber of levels is two, i.e., K=2, but the inventive concept is not solimited. For example, simultaneous demodulator/decoder 320 provides ULsignal 321-1 and LL signal 321-2. The former ideally represents what wastransmitted on the upper layer, i.e., signal 4-1 of FIG. 3; while thelatter ideally represents what was transmitted on the lower layer, i.e.,signal 4-2 of FIG. 3.

Turning now to FIG. 8, an illustrative block diagram of simultaneousdemodulator/decoder 320 is shown. Unified demodulator/decoder 320comprises UL demodulator 330, UL decoder 335, log likelihood ratio (LLR)look-up table (LUT) 570 and LL decoder 340. Hierarchical signal 311 isapplied to UL demodulator 330, which demodulates this signal andprovides therefrom a demodulated UL signal as represented by demodulatedUL signal point stream 333. Referring now to FIG. 9, an illustrativeblock diagram of UL demodulator 330 is shown. UL demodulator 330includes digital resampler 415, matched filter 420, derotator 425,timing recovery element 435 and carrier recovery element 440.Hierarchical signal 311 is applied to digital resampler 415, whichresamples hierarchical signal 311 using UL timing signal 436, which isprovided by timing recovery element 435, to provide resampledhierarchical signal 316. Resampled hierarchical signal 316 is applied tomatched filter 420. The latter is a band-pass filter for filteringresampled hierarchical signal 316 about the UL carrier frequency toprovide a filtered signal to both derotator 425 and the above-mentionedtiming recovery element 435, which generates therefrom UL timing signal436. Derotator 425 derotates, i.e., removes the carrier from thefiltered signal to provide a demodulated UL signal point stream 333.Carrier recover element 440 uses the demodulated UL signal point stream333 to recover therefrom UL carrier signal 332, which is applied toderotator 425.

Referring back to FIG. 8, UL decoder 335 acts in a complementary fashionto corresponding UL encoder 105 of transmitter 5 and decodes thedemodulated UL signal point stream 333 to provide UL signal 321-1. Asnoted above, UL signal 321-1 represents the data conveyed on the upperlayer, e.g., as represented by signal 4-1 of FIG. 3. It should beobserved that UL decoder 321-1 recovers the data conveyed in the UL by,in effect, treating the LL signal as noise on the UL signal. In otherwords, EL decoder 321-1 operates as if UL signal 321-1 representssymbols selected from signal space 89 of FIG. 4.

Turning now to the LL signal, and in accordance with the principles ofthe invention, UL signal point stream 333 is also provided to LLR LUT570. UL signal point stream 333 is a stream of received signal points,each received signal point having an in-phase (I_(REC)) component (572)and a quadrature (Q_(REC)) component (571) in a signal space. This isfurther illustrated in FIG. 10 for a received signal point z, where;z=I _(rec) +jQ _(rec).   (1)

The I_(REC) and Q_(REC) components of each received signal point areapplied to LLR LUT 570. The latter stores a LUT 599 of precomputed LLRvalues as illustrated in FIG. 11. In particular, each row of LUT 599 isassociated with a particular I component value (an I row value), whileeach column of LUT 599 is associated with a particular Q component value(a Q column value). LUT 599 has L rows and J columns. LLR LUT 570quantizes the I_(REC) and Q_(REC) component values of a received signalpoint to form an input address, which is used as an index into LUT 599for selecting therefrom a respective precomputed LLR. Each symbolinterval, T, the selected LLR is provided via signal 396 to LL decoder340. For example, if the I_(REC) component value is quantized to thefirst row and the Q_(REC) component value is quantized to the firstcolumn, then LLR 598 would be selected and provided via signal 396 to LLdecoder 340 of FIG. 8.

Other than the inventive concept, and as known in the art, for a givenbit-to-symbol mapping M(b_(i)), where M are the target symbols and bi=0,1, . . . B-1, are the bits to be mapped where B is the number of bits ineach symbol (e.g., B may be two bits for QPSK, three bits for 8-PSK,etc.), the log-likelihood ratio function for the ith bit of a B bitvalue is:LLR (i, z)=log [(prob(b _(i)=1|z))/(prob (b _(i)=0|z))];   (2)where b_(i) is the ith bit and z is the received signal point in thesignal space. The notation “prob (b_(i)=1|z)” represents the probabilitythat the ith bit is a “1” given that the signal point z was received.Similarly, the notation “prob (b_(i)=0|z)” represents the probabilitythat the ith bit is a “0” given that the signal point z was received.

For a two-dimensional signal space, the probabilities within equation(2) are assumed to be based upon additive Gaussian white noise (AWGN)having a probability density function (PDF) of: $\begin{matrix}{{{prob}(n)} = {\frac{\exp( \frac{- {n}^{2}}{2\sigma^{2}} )}{2{\pi\sigma}^{2}}.}} & (3)\end{matrix}$Therefore, the LLR for a given bit and received signal point are definedas: $\begin{matrix}{{{LLR}( {i,z} )} = {{\log\lbrack \frac{\sum\limits_{M_{{{bit}\quad i} = 1}}{\exp( \frac{- {{z - M}}^{2}}{2\sigma^{2}} )}}{\sum\limits_{M_{{{bit}\quad i}\quad = \quad 0}}{\exp( \frac{- {{z\quad - \quad M}}^{2}}{2\quad\sigma^{2}} )}} \rbrack}.}} & (4)\end{matrix}$It can be observed from equation (4) that the LLR for a given receivedsignal point z is a function of z, the target symbols M, and the rmsnoise level σ. An LLR is also one example of a “soft metric.”

A pictorial illustration of the calculation of an LLR ratio is shown inFIGS. 12 and 13. FIG. 12 shows an illustrative symbol constellation. Forsimplicity a 4 symbol QPSK (quadrature phase shift keyed) constellationis shown, however, it should be noted that other sizes and shapes ofsymbol constellations could also have been used, e.g., 3 bits for 8-PSK,4 bits for 16-QAM, a hierarchical 16-QAM, etc. As can be observed fromFIG. 12, there are four symbols in the signal space 89, each symbolassociated with a particular two bit mapping [b1, b0]. Turning now toFIG. 13, a received signal point z is shown in relation to the symbolsof signal space 89. It can be observed from FIG. 13 that the receivedsignal point z is located at different distances d_(i) from each of thesymbols of signal space 89. For example, the received signal point z islocated a distance d₄ from the symbol associated with the two bitmapping “01.” As such, the LLR(b0) is:ln[(probability b0 is one)/(probability b0 is zero)]; or   (5A)ln[(probability (symbol 01 or 11))/(probability (symbol 00 or 10))]; or  (5B)ln[{exp(−d₄ ²/(2σ²))+exp(−d₃ ²/(2σ²))}/{exp(−d₂ ²/(2σ²))+exp(−d₁²/(2σ²))}].   (5C)while the LLR(b1) is:ln[(probability b1 is one)/(probability b1 is zero)]; or   (6A)ln[(probability (symbol 10 or 11))/(probability (symbol 00 or 01))]; or  (6B)ln[{exp(−d₁ ²/(2σ²))+exp(−d₃ ²/(2σ²))}/{exp(−d₂ ²/(2σ²))+exp(−d₄²/(2σ²))}].   (6C)

Returning to FIG. 8, it can be observed that LLR LUT 570 (i.e., LUT 599)is initialized to a set of hierarchical LLR values 573. In accordancewith the principles of the invention, these are calculated a priori withrespect to the combined symbol constellation such as the one illustratedin FIG. 5 and shown again in FIG. 14. In other words, the LLRs for theLL are determined—not with respect to the LL signal space (e.g., signalspace 89 of FIG. 4)—but with respect to the combined signal space (e.g.,signal space 79 of FIG. 5). For every received signal point z, adistance between each symbol of signal space 79 and the received signalpoint z is determined and used in calculating an LLR. For simplicity,only some of these distances, d_(i), are shown in FIG. 14. Thehierarchical LLR values 573 can be formed in any number of ways. Forexample, receiver 30 may perform the calculations by using, e.g., atraining signal, provided by transmitter 5 either during start-up, orre-initialization, of communications between the two endpoints(transmitter 5 and receiver 30 ). As known in the art, a training signalis a predefined signal, e.g., a predefined symbol sequence that is knowna priori to the receiver. A predefined “handshaking” sequence mayfurther be defined, where the endpoints exchange signaling beforecommunicating data therebetween. Alternatively, the calculations may beperformed remotely, e.g., at the location of transmitter 5 and sent toreceiver 30 via an in-band or out-of-band signaling channel (this couldeven be via a dial-up facility (wired and/or wireless) (not shown)).

Referring back to FIG. 8, LL decoder 340 receives the sequence of LLRs(the soft input data), via signal 396, and provides therefrom LL signal321-2. LL decoder 340 operates in a complementary fashion to that of LLencoder 110. It should also be noted that LL decoder 340 may also be asoft-input-soft-output decoder, and provide soft output values, whichare then additionally processed (not shown) to form LL signal 321-2.

Thus, and in accordance with the principles of the invention, receiver30 directly determines the LL signal from a received hierarchicalmodulation based signal. This is referred to herein as a simultaneousmode of decoding. In particular, the UL signal point stream 333 isprocessed to generate soft input data, e.g., LLRs, to recover therefromthe LL data. In other words, receiver 30 processes the receivedhierarchical modulation based signal such that the UL and the LL areprocessed independently of each other.

Attention should now be directed to FIG. 15, which shows an illustrativeflow chart in accordance with the principles of the invention of aprocess for use in receiver 30 of FIG. 1. In step 605, receiver 30begins (or restarts) communications with transmitter 5 and receives apredefined hierarchical modulation based training signal comprisingpredefined UL symbols and LL symbols. In step 610, receiver 30calculates hierarchical LLRs from the received training signal withrespect to the combined signal space 79. In step 615, receiver 30 storesthe calculated hierarchical LLRs in LLR LUT 570. Finally, in step 620,receiver 30 switches to a data communications mode and begins to receivedata transmitted from transmitter 5 of FIG. 1.

It should be noted that although the above-described embodimentdescribed LL decoder 340 as receiving soft metrics, LL decoder 340 mayreceive signal points as represented by signal point stream 333 and, assuch, further process the received signal point data to derive therefromLLRs as described above, e.g., LLR decoder 340 includes LLR LUT 570.Conversely, UL demodulator 330 may be modified to include therein LLRLUT 570 for providing the soft metrics to LL decoder 340.

Another illustrative embodiment of the inventive concept is shown inFIG. 16. However, only those portions relevant to the inventive conceptare shown. For example, analog-digital converters, filters, decoders,etc., are not shown for simplicity. In this illustrative embodiment anintegrated circuit (IC) 705 for use in a receiver (not shown) includessimultaneous demodulator/decoder 320 and at least one register 710,which is coupled to bus 751. The latter provides communication to, andfrom, other components of the receiver as represented by processor 750.Register 710 is representative of one, or more, registers of IC 705,where each register comprises one, or more, bits as represented by bit709. The registers, or portions thereof, of IC 705 may be read-only,write-only or read/write. In accordance with the principles of theinvention, simultaneous demodulator/decoder 320 simultaneously decodes areceived hierarchically modulated signal and at least one bit, e.g., bit709 of register 710, is a programmable bit that can be set by, e.g.,processor 750, for controlling this operating mode. In the context ofFIG. 16, IC 705 receives an IF signal 701 for processing via an inputpin, or lead, of IC 705. A derivative of this signal, 311, is applied tosimultaneous demodulator/decoder 320. The latter provides output signals321-1 through 321-K as described above. Simultaneous demodulator/decoder320 is coupled to register 710 via internal bus 711, which isrepresentative of other signal paths and/or components of IC 705 forinterfacing simultaneous demodulator/decoder 320 to register 710 asknown in the art.

In view of the above, it should also be noted that although described inthe context of a receiver coupled to a display as represented by TV 35,the inventive concept is not so limited. For example, receiver 30 may belocated further upstream in a distribution system, e.g., at a head-end,which then retransmits the content to other nodes and/or receivers of anetwork. Further, although hierarchical modulation was described in thecontext of providing communication systems that are backward compatible,this is not a requirement of the inventive concept. It should also benoted that groupings of components for particular elements described andshown herein are merely illustrative. For example, either or both ULdecoder 335 and LL decoder 340 may be external to element 320, whichthen is essentially a demodulator that provides a demodulated signal.

As such, the foregoing merely illustrates the principles of theinvention and it will thus be appreciated that those skilled in the artwill be able to devise numerous alternative arrangements which, althoughnot explicitly described herein, embody the principles of the inventionand are within its spirit and scope. For example, although illustratedin the context of separate functional elements, these functionalelements may be embodied on one or more integrated circuits (ICs).Similarly, although shown as separate elements, any or all of theelements of may be implemented in a stored-program-controlled processor,e.g., a digital signal processor (DSP) or microprocessor that executesassociated software, e.g., corresponding to one or more of the stepsshown in FIG. 15. Further, although shown as separate elements, theelements therein may be distributed in different units in -anycombination thereof. For example, receiver 30 may be a part of TV 35. Itis therefore to be understood that numerous modifications may be made tothe illustrative embodiments and that other arrangements may be devisedwithout departing from the spirit and scope of the present invention asdefined by the appended claims.

1. A method for use in a receiver, the method comprising: receiving ahierarchical modulation based received signal, the hierarchicalmodulation based signal comprising at least a first signal layer and asecond signal layer; and simultaneously recovering from the receivedhierarchical modulation based received signal data conveyed in the firstsignal layer and data conveyed in the second signal layer.
 2. The methodof claim 1, wherein the first signal layer is an upper signal layer andthe second signal layer is a lower signal layer.
 3. The method of claim1, wherein the simultaneously recovering step includes the steps of:decoding the hierarchical modulation based signal to recover dataconveyed in the first signal layer; generating soft metrics from thehierarchical modulation based signal as a function of a combined signalspace of the hierarchical modulation based signal; and decoding thehierarchical modulation based signal to recover data conveyed in thesecond signal layer as a function of the generated soft metrics.
 4. Themethod of claim 3, wherein the soft metrics are log-likelihood ratios.5. The method of claim 3, wherein the combined signal space is acombination of a signal space of the first signal layer and a signalspace of the second signal layer.
 6. The method of claim 3, wherein thegenerating step includes the step of using the hierarchical modulationbased signal as an index into a look-up table of soft metrics.
 7. Amethod for use in a receiver for receiving a hierarchical modulationbased signal comprising at least a first signal layer and a secondsignal layer, the method comprising: receiving a training signal from anendpoint; calculating soft metric values as a function of a combinedsignal space and the received training signal, wherein the combinedsignal space is a combination of a signal space of the first signallayer and a signal space of the second signal layer; and storing thecalculated soft metric values in a look-up table.
 8. The method of claim7, further comprising: receiving the hierarchical modulation basedsignal; decoding the hierarchical modulation based signal to recoverdata conveyed in the first signal layer; and decoding the hierarchicalmodulation based signal to recover data conveyed in the second signallayer as a function of the stored metric values.
 9. The method of claim8, wherein the soft metric values are log-likelihood ratios.
 10. Areceiver comprising: a demodulator for demodulating a received signal toprovide a hierarchical modulation based signal comprising at least twosignal layers; a first decoder operative on the hierarchical modulationbased signal for decoding one of the at least two signal layers toprovide data therefrom; and a second decoder for providing data from theother of the at least two signal layers; wherein the second decoderoperates independently of the first decoder.
 11. The apparatus of claim10, wherein the at least two signal layers include an upper signal layerand a lower signal layer.
 12. The apparatus of claim 10, furtherincluding a look-up table for storing therein soft metrics, wherein thesoft metrics are determined as a function of a combined signal space ofthe at least two signal layers and wherein the look-up table providesthe soft metrics to the second decoder for use therein for providing thedata from the other of the at least two signal layers.
 13. The apparatusof claim 12, wherein the soft metrics are log-likelihood ratios. 14.Apparatus comprising: a television set for displaying video content; anda receiver coupled to the television set for receiving a hierarchicalmodulation based signal conveying the video content, wherein thereceiver simultaneously decodes at least two signal layers of thereceived hierarchical modulation based signal for providing the videocontent to the television set. 15 The apparatus of claim 14, wherein thereceived hierarchical modulation based signal is a satellite signal. 16.The apparatus of claim 14, wherein the receiver includes a look-up tablefor storing soft metrics, which are determined as a function of acombined signal space of the at least two signal layers.
 17. Theapparatus of claim 16, wherein the soft metrics are log-likelihoodratios.
 18. Apparatus comprising: a simultaneous demodulator/decoder forprocessing a hierarchical modulation based received signal comprising atleast a first signal layer and a second signal layer; and at least oneregister for use in controlling the simultaneous demodulator/decoder forsimultaneously recovering from the hierarchical modulation basedreceived signal data conveyed in the first signal layer and dataconveyed in the second signal layer.
 19. Apparatus comprising: a leadfor receiving a hierarchical modulation based received signal comprisingat least a first signal layer and a second signal layer signal; and asimultaneous demodulator/decoder for processing the hierarchicalmodulation based received signal for simultaneously recovering therefromdata conveyed in the first signal layer and data conveyed in the secondsignal layer.